[JavascriptFlowtyped] Adding parent flow type to child type using "& ParentType" to allow flow js access parent fields from child types

Created by: GrizliK1988

Issue: https://github.com/OpenAPITools/openapi-generator/issues/4262

Javascript Flowtyped generator does not add parent type to created types.

Example how it is now:

type ChildType = { ... }

type ParentType = { ... }

What's done:

type ChildType = { ... } & ParentType

type ParentType = { ... }

In this case Flow will not complain when client code will access fields from ParentType if object has ChildType.

@CodeNinjai (2017/07) @frol (2017/07) @cliffano (2017/07) What do you think?